<p>
  <b>Project</b><br />
  <div>
  <%= form.select :project_id, projects  %>
  </div>
</p>

<p>
  <b>Milestone</b><br />
  <div id="milestones">
  <%= form.select :milestone_id, milestones %>
  </div>
  <%= observe_field :burndown_project_id,
                    :frequency => 0.5,
                    :update => :milestones,
                    :url => { :controller => "Milestones", 
                              :action => :index} %>
</p>